Sofiya Vlacic was a resident of Odessa, Ukraine, and the sister of Petro and Roman. Sofiya entered a romantic relationship with Jack Bauer when he lived in Odessa as "Borys" while hiding from the U.S. and Russian governments.

One morning, Jack cooked breakfast for the two of them before he left for work at the port. She joked that his Russian seemed to be getting worse, and insisted that he refrain from butchering her native tongue so early in the morning. As Jack departed, Sofiya received a phone call from her mother, and warned him to leave before he got sucked into one of her mother's lectures on how Sofiya needed to start having children before her forties.

That evening, Jack called Sofiya to tell her that he was helping Petro with a work-related problem. She said that her brother Roman had been missing since the previous night, but Jack assured her that he would turn up eventually. (Underground #1)